Recently the demand for a commuter helicopter which lands and takes off in an urban heliport is increasing, and noise reduction is required for realizing such a commuter helicopter. One of effective countermeasures is to slow down the rotating speed of the main rotor.
FIG. 10 is a perspective view showing an example of a conventional power transmission mechanism for a helicopter. The main rotor is fixed on an extension of a main rotor shaft 14, and a tail rotor is fixed on an extension of a tail rotor shaft 18. Output shafts of a pair of engines 1, 2 rotate at 20,000 to 30,000 rpm, whose rotation speed is reduced to about 6,000 rpm by gear boxes 3, 4. The output shafts are coupled to spiral bevel gears 7, 8 through freewheel clutches 5, 6 and are further in mesh with one collector gear 13, thereby rotating and driving the main rotor shaft 14 at about 350 rpm. Relating to peripheral accessories, a lubricant pump is driven by spiral bevel gears 7, 8, and a hydraulic pump is driven through transmission shafts 9, 10, and a cooling fan is driven through a transmission shaft 11.
On the other hand, a gear 15 is in mesh with the collector gear 13 and coupled with three tail rotor shafts 16, 17 and 18 so as to divide the torque of the collector gear 13, thereby rotating and driving the tail rotor at about 2,200 rpm.
As other prior arts, Japanese Unexamined Patent Publications JP-A 4-287799(1992), JP-A 4-306196(1992), JP-A 5-139386(1993), and JP-A 5-149351(1993) are known.
In conventional helicopters, since revolution of the engines 1, 2 is slowed down at a fixed reduction ratio by different gear trains, the main rotor and tail rotor can rotate only at a constant number of rotations.
Accordingly, in order to decrease the rotating speed of the main rotor to reduce noises, it is necessary to decrease a number of revolutions per unit time of the engine. However, since a conventional engine is designed so as to attain its maximum performance at a predetermined number of revolutions of the engine, it is difficult to control the number of revolutions of the engine unrestrictedly, and the number of revolutions can be lowered by at most only about 3%. Additionally, it may be considered to control the number of revolutions of the engine by optimizing the fuel flow rate and ignition timing by electronic control of the engine, but because of other problems such as lowered efficiency, generation of resonance and limitation of allowable strength of transmission system, at the present the number of revolutions of the engine can be varied by only a small percent in practice. Besides, if the number of rotations of the main rotor is decreased to a large extent, the lift may drop, and the flight performance of the helicopter may be lowered.
It is hence a more practical manner to decrease the number of rotations of the main rotor over urban districts to reduce noises, and return the number of rotations of the main rotor to an optimum over rural districts so as to fly at its maximum performance. For example, it is reported that the equivalent weight noise level is lowered by 3 dB to 5 dB by decreasing the number of rotations of the main rotor or the wing end speed by 10%.